Photovoltaic applications—photovoltaic solar cells—require silicon of very high purity, designated by the expression “SoG-silicon” (SoG being the acronym of “Solar Grade”). Typically the impurity content should be of the order of 10−6 to 10−5, i.e. of the order of 1-10 ppm. This is an average content, all the elements included in the silicon not all having the same adverse effect. For example, the tungsten content (W) should be less than 0.01 ppm, that of phosphorus (P) less than 5 ppm and the boron (B) content less than 1 ppm.
In order to obtain silicon of this quality, the use of electronic grade silicon waste is known. However, industrial demand for silicon with photovoltaic quality has become so high that these wastes are in an insufficient amount in order to meet the demand from the market; this shortage is notably expressed by a strong increase in the prices of photovoltaic grade silicon.
Moreover there exists a metallurgical process consisting of gasifying silicon, of distilling it so as to form chlorosilane which should then be converted into the silane (SiH4), and then subjecting it to cracking, with which silicon powder may thereby be obtained. However, this process is characterized by a large number of operations and by emission of chlorine during the conversion of chlorosilane into silane. Further, the quality of the metallurgical silicon is less than the quality required for photovoltaic applications.
Methods have therefore been developed which consist of submitting liquid silicon to the flux of a plasma torch in which reactive gases are added capable of causing volatilization of the impurities. The reaction occurs at the liquid-plasma interface and requires rapid renewal of the gases and of the liquid at this interface. This renewal is obtained by means of electromagnetic mixing or mixing by injecting gas into the liquid silicon. However, this is a discontinuous method, so-called batch processing. Silicon is indeed contained in a crucible. The duration of the processing directly depends on the free surface and on the volume of crucible.
In order to reach a quasi-continuous method, document JP11-209195 proposes the cascading of several silicon crucibles. Each of these crucibles is equipped with electron guns, the function of which is to ensure heating of the silicon and evaporation of the phosphorus. This solution, is however, not optimum since the silicon is not renewed at the surface, the efficiency (i.e. the purification rate) decreases rapidly, and any mixing causes mixing between non-purified silicon and purified silicon inside a same crucible.
By multiplying the number of crucibles it is possible to reduce this difficulty but it makes the method complex. It requires in particular many costly handling operations. Moreover it multiplies the sources, electron guns or plasma torches.
One of the objects of the invention is therefore to propose a method for purifying silicon which allows suppression of the majority of the handling operations. Another object of the invention is to guarantee the absence of contamination of purified silicon by non-purified silicon.